Electromagnetic interference (EMI) can arise in any electronic system, either directly from circuitry or indirectly by conduction along connecting cables and by radiation. EMI can also arise from external sources to create problems in various systems. Undesirable signals occur whenever interference has a source, a receiver, and a transfer path. Accordingly, EMI can be reduced or eliminated by suppressing interference at the source, protecting the receiver against interference, and by reducing transmission.
Interference can propagate by radiation of electromagnetic waves in free space and by conduction on a conductive pathway. Techniques for suppressing radiant interference include shielding with conductive materials such as conductive adhesive tapes, wire mesh, and gaskets, or absorbing with absorbing materials such as carbon or ferrite-filled binders. Techniques for suppressing conducted interference include usage of ferromagnetic cable absorbers, connector backshields, filtered connectors, ferrite toroids, and feedthrough capacitors that all reduce emissions conducted onto connecting cables. Electromagnetic compatibility is regulated throughout the world. European Norms (EN) defines regulations applicable in all European Union (EU) and European Free Trade Associated (EFTA) countries. Federal Communications Commission (FCC) regulates electromagnetic compatibility in the United States.
Fundamentally, EMI should be addressed using good design practice to eliminate interference in design requirements. Design practice may be ineffective for interference that is directly related to inherent operating principles and for interference that is not detected until the final design phase. Additional suppression may be needed using extra suppression components such as ferrites, capacitors, or shielding elements.
A particular EMI emission difficulty is excessive radiated emissions originating from 10/100/1000 BaseT unshielded cable for cables connected to computer servers or other local area network (LAN) based equipment. Generally, LAN cable radiation can be the single most significant source of aberrant radio noise from servers and LAN devices. Common mode energy from non-LAN sources within a server often couples to LAN circuitry and is conducted to the outside environment through unshielded LAN cables.
Excessive radiation from LAN connectors and cables are generally addressed using three techniques. First, the offending noise source can be reduced inside the system enclosure of a LAN device, a technique that has several difficulties. The interior noise source can be hard to localize and, once localized, hard to remedy. Often an attempted fix will fail to accomplish significant noise reduction and can worsen the problem. An attempted fix that improves performance in prototype form can be ineffective when applied in a manufacturing setting. Due to the difficulty in detecting and treating a noise source or sources, numerous design or update cycles may be required to attain improvement. Attempts to reduce EMI inside the enclosure can deteriorate into a trial and error strategy that is impractical with a limited number of design cycles and large cost differentials of various fixes such as more robust system connectors and larger printed circuit board (PCB) layer counts. Addressing EMI inside the system enclosure presupposes that a design team will recognize the magnitude of the noise problem at time of conception and design, typically an incorrect presumption in practice.
Second, filters either integrated into connectors or on a system printed circuit board near LAN magnetics can limit the noise level, an approach that is generally limited by extreme consequences of LAN parametrics, resulting in a lack of commercially viable design choices. Furthermore, LAN magnetics vendors do not typically have the technological understanding to appropriately design filters capable of attenuating high frequency common mode noise.
Third, global usage of shielded LAN cables can avert LAN cable radiation but is economically prohibitive. To comply with the specification for every cable to be shielded, a customer would need to replace the entire LAN cable infrastructure with new and expensive shielded LAN cables, easily an expense of multiple thousands of dollars for relatively large customers.